Fuel Economy Tip – Roll the Windows Up

With the weather being nice, I’m sure many of you wouldn’t mind hopping in the car, rolling down the windows and going for a drive. While that’s not a bad idea, you may want to keep the windows up if you’re trying to get good gas mileage out of your leisurely drive.

Keep your car’s windows up, especially the faster you go.

A car is its most aerodynamic (meaning it moves through the air with less resistance) when the windows are rolled up.

When your vechicle’s windows are down, especially at highway speeds, there is more drag placed on the vehicle. Drag is the force that opposes your car’s movement, so the less amount of drag, the less amount of energy (gas) your car has to use to get up to and maintain speed.

By having the windows down at highway speeds, you can reduce your vehicle’s fuel economy by up to 3%, or roughly add $.09 to each gallon of gas you use! For a 15 gallon car that is filled up once per week, that adds up to over $70 per year.

Especially when the weather is nice (not hot), you should be able to keep the windows up and get away with using a cool vent.

You really can’t make a blanket statement regarding a/c on or off vs. windows up or down. Which is more fuel efficient depends upon the vehicle and its speed. Generally, the faster the vehicle is going, the more likely it will be that having the a/c on is more fuel efficient: aerodynamic drag increases as the square of speed. Equally important, though, is the engine’s power capability: the increase in power required for a/c with a powerful engine will be quite small as a percentage of power, and therefore any increase with such an engine will likely not make much difference. The opposite is true with low-power engines. Turning on the a/c in my 130 hp / 129 ft-lbs torque 2.0 liter four cylinder very noticeably slows the care down!

I’m extremly curious about this subject, and wish more explinatinon was available.

First of all… when you’re on the highway, who has ALL the widowns down?
What if you have 1 windown down 1 inch?
How about 1 windown down 2 inch?
How about 0.00001 inch?

I’d like to know some details of the study before i buy in.

And what size of Car? a honda civic or a Ford Explorer?
Does it make a difference?

How about A/C Possition?
If you have A/C on very low or very high?

I’m 100% sure all these effect details are not minor and have a significant effect on the car.
IMO there should be a study which DETAILS each speed, with the ideal driving condition. (example; driving at 65mph in honda civic = drive with windows all the way up and a/c on low)

Uh, Jeff, are you sure about this. According to the chart you provided — which gives good data by the way — it shows that gas consumption increases when a car’s windows are down vs. when they are up. I’m not exactly sure why you think the tip of “keep your car windows up” is wrong.

The graph shows as speed increases gas consumed also increases. So like Ayall said, the worst you can do for your gas mileage is have your windows up and ac on (I guess some people have their windows down with ac on but that wasn’t tested), followed by windows down ac off, and the best is windows up ac off. It gets really bad over 80kph and takes off from there. Keep in mind, they were testing an suv at 30 deg C. These results are only for this situation, it may vary with other cars.

There was also a study using a sedan @ 30degC in the pdf i linked from the site you provided.

That study concluded same results, “that the best is AC OFF windows UP, followed by AC OFF windows DOWN, followed by AC ON (Medium Power) windows UP,” but it was a much closer race for “AC OFF windows DOWN vs. AC ON windows UP.”

If i’m reading the graphs correct, it looks like they tested at three speeds all having the same results (that the best is AC OFF windows UP, followed by AC OFF windows DOWN, followed by AC ON (Medium Power) windows UP).
50kph = 30 mph
80kph = 50 mph
110kph = 70 mph

That’s a great study then, wow! It’s funny that you’ll see many other websites (including this one) claim that it’s better to leave your windows up based on no factual evidence (or very little). The treehugger study and the mythbusters study are the other two I have seen that have some research involved and those are the two that show ac off windows up is the best. I do appreciate this blog site letting us post comments though.

Trackbacks

[…] Driving on the highway with their windows down to avoid using the air conditioner. We’ve become accustomed to thinking that our car’s AC is the biggest drain on our car’s gas mileage. While using it will reduce your gas mileage, there are many instances where using your AC instead of rolling down the windows will actually be better for your car’s gas mileage. One of the biggest drains on your car’s gas mileage is aerodynamic drag – something that is exacerbated by rolling down the windows. If you’re going to drive on the highway, go ahead and turn on your AC and keep the windows up! […]